Lecture 15 Flashcards
What features does our ventilation system need to create constant air flow?
⦁ Ventilation requires a pressure gradient which generates the airflow.
this needs control centres tha work (brain stem) to allow for rhythm / variation, muscles which don’t get easily fatigued or get paralysis, lungs which have good compliance and airways that are not too narrow (so that resistance isn’t too high).
What functions do the muscles of the respiratory system need to be able to overcome?
⦁ The muscles must expand the chest/lungs (stiffness, elasticity of lung, chest wall and the effects of surface tension), overcome the lung stiffness and move the air, overcoming the airways resistance (friction).
These provide the work (energy requirements) of breathing.
What is lung compliance? What are two associated diseases?
A measure of how easy it is to change the lung volume. It changes with the pressure being changed to and from and gets harder as the pressure rises, it is a non linear plot.
Emphysema is too high compliance (due to destruction of elastic tissue), Fibrosis is when compliance is too low.
How is surface tension in the lungs caused? How do our lungs deal with this? What happens without this?
⦁ The fluid lining the alveoli exerts surface tension, causing them to contract (resist expansion), surfactant helps to reduce the force required to expand the lungs due to this surface tension. Without this surfactant we would struggle to expand our lungs and alveoli can collapse or fill with fluid.
What is respiratory distress syndrome?
Prematurely born infants lack surfactant (produced after 6 months as a foetus) and have great difficulty inflating lungs. This leads to respiratory distress syndrome (can be gotten by adults, typically with significant trauma). Those with this will not have the typical wire like structure of the lungs due to the collapsed alveoli.
What equation is linked too airway resistance and what does this mean?
⦁ Air flow is mainly laminar during quiet breathing, resistance can be determined by Poiseuille’s law R = 8nl/pi x r^4 where n = viscosity, l = length and r = radius. This means bronchoconstriction and bronchodilation are important elements in airways resistance and asthma can have a huge effect.
Where does most airway resistance arise?
⦁ Most airflow resistance arises within the first 6 generations of the lower airway (bronchi), the small airways contribute very little to airways resistance. This is because a block higher up will affect all airways afterwards.
What are the main two measurements of lung function?
Measurements of lung function are spirometry (lung volume (how much) and air flow rates (how fast it can be shifted in and out)) and also can be measured by peak expiratory flow rates.
How does spirometry work? What are the main values?
Spirometers function by bbreathing into and out of a hollow bell inverted over water, it measures the volume inspired and exhaled (it can measure how much and how fast, is a common simple test which can be mechanical or digital and can provide a test response to therapy).
The volumes which can be measured are tidal volume (volume of exhaled air after normal inspiration, typically about 500 ml), inspiratory reserve volume (maximum breath in compared to tidal, typically about 3000 ml), expiratory reserve volume (maximum expiratory breath compared to tidal, is typically less than the inspiratory reserve volume, typically about 1500 ml). There is also left over residual air volume below the expiratory reserve volume of about a litre which can’t be removed (the lungs are never empty).
Typical total lung volume is about 6 litres and is made up of all these values.
Vital capicity is made up of the expiratory reserve volume, tidal volume and inspiratory reserve volume.
Functional residual capcity is the typical volume of air left in the lungs at the end of a normal (tidal) breath out
How do we measure residual volume?
To measure residual volume we use helium dilution, it can’t be measured by spirometry..
How do we measure flow of air with spirometry? What ratio is important for this and what can it tell us.
Forced measurements give info about flow of air (maximal breath in, maximal breath out (FVC) as hard as possible). This gives us the forced expiratory volume (FEV1) in one second (this must be taken from the first section). The FEV1 /FVC ratio is normally 80%, if it is below 70% that indicates airway. obstruction.
How can we get predicted spirometry values?
Normal/predicted values for spirometry will vary depending on age gender and height. These are combined in a graph and can be used to get predicted values.
What factors can spirometry identify?
Spirometry can identify increased airway resistance (e.g asthma) or decreased total lung capacity (e.g from structural/functional problems).
What is the difference between obstructive and restrictive diseases? Do all lung diseases have to be like this?
Spirometry can identify increased airway resistance (e.g asthma) or decreased total lung capacity (e.g from structural/functional problems).
Not all have to be in these two categories, there are others.
What is PEFR and how is it useful?
peak expiratory flow rates give information about airways resistance and can be done using portable devices or spirometry and tells us about airway resistance, we determine the “normal” values from nomogram (done by the same person), the real value comes from when an individual’s PEFR change from the normal value, if it decreases that suggests decreased flow.
